The Environmental Protection Agency plans to change the way it allows automakers to group similar vehicles into "families" for gas mileage ratings, a top official told USA TODAY Thursday.

"We're going to update general-labeling regulations," says Chris Grundler, director of the EPA's Office of Transportation and Air Quality. The update will cover "which vehicles need to be tested."

He also says the EPA is dramatically increasing its vehicle testing, adding 30 more workers to its examination program. That's about a 40% increase. At present, automakers test their vehicles and come up with fuel-economy figures based on EPA rules. EPA audits them on a spot-check basis. "I can't underscore how important is it to EPA that we protect the integrity of these (fuel economy estimate) labels," he says.

The decision to seek a change in the regulations is an outgrowth of the relabelling of the 2013 Ford C-Max. After an outcry, Ford said Thursday that it will lower the gas mileage of the hybrid crossover to 43 miles per gallon in combined city and highway driving, down from 47 mpg.

Ford followed the rules in testing the C-Max and assigning a gas mileage figure to it, says Grundler. Longstanding rules allow automakers to group similar vehicles into categories based on similar engines, transmissions and weight class. But the system isn't working as well now with today's high-mileage hybrids and plug-ins.

"Historically this worked fine," he says. "What we're discovering now is that this the first time we're seeing a family of hybrid vehicles" in which the procedure isn't working as well.

Luckily, he says, only there are only two groupings so far -- from Toyota and Ford. Toyota chose to test each of its Prius models separately, avoiding the revision that faces Ford. "The good news for the buying public is that this is an emerging issue."

He says any rule change will be subject to discussion by the agency, automakers and all other interested parties.